A typical situation in which the present invention finds significant utility is that involving an agricultural machine having a main frame which carries a harvesting part or header for movement between raised and lowered positions. Conventionally, a hydraulic cylinder and piston unit is used as the force-exerting means, the cylinder and piston rod being connected respectively to the frame and header. During normal operation, the hydraulic unit is retracted to achieve the lowered or operating position of the header. During transport of the machine, or for purposes of repairs, etc., the header is raised by extension of the hydraulic unit. When the header is raised, it is desirable that some means be provided to prevent accidental descent of the header, which could cause serious injury to a mechanic working below the raised header. During transport, it is desirable that the hydraulic fluid be relieved of the duty of supporting the raised header.